We plan to examine the role of catecholamine-containing cells in parasympathetic ganglion cells in the heart of the mudpuppy. Structural and histochemical studies have shown that in sympathetic ganglia one finds in addition to the well-studied large neurons (principal cells) also smaller cells that contain catecholamines. The exploration of the physiological role of these cells has been hindered in the past because they are not readily accessible for direct probing. A preparation that seems suitable for physiological studies has recently been described by McMahan and Purves (1976) and by Roper (1976) in the mudpuppy, Necturus maculosus. It consists of a transparent septum in which one recognizes in the live preparation both cholinergic principal cells that innervate the heart and catecholamine-containing interneurons. The principal cells receive excitatory innervation from vagus fibers and another set of synapses from the interneurons. We intend to analyze the action of these two chemically diverse synapses in terms of (1) the synaptic potentials they generate, (2) the physiologial characteristics and the distribution of cholinergic and catecholaminergic receptors on the surface membrane of principal cells, and (3) integration at the cellular level when the two inputs are activated.